A prior art accelerometer with high performance potential is described in U.S. Pat. No. 3,702,073. The accelerometer comprises three primary components, a proof mass assembly, and upper and lower stators or magnetic circuits between which the proof mass assembly is supported. The proof mass assembly includes a movable paddle that is suspended via flexures to an outer annular support ring, such that the paddle can pivot with respect to the support ring. The paddle, flexures and support ring are commonly provided as a unitary structure composed of fused quartz.
Both upper and lower surfaces of the paddle include capacitor plates and force balancing coils. Each force balancing coil is positioned on the paddle such that the central axis of the coil is normal to the top and bottom surfaces of the paddle, and parallel the sensing axis of the accelerometer. A plurality of mounting pads are formed at spaced-apart positions around the upper and lower surfaces of the annular support ring. These mounting pads mate with inwardly facing surfaces of the upper and lower stators when the accelerometer is assembled.
Each stator is generally cylindrical, and has a bore provided in its inwardly facing surface. Contained within the bore is a permanent magnet. The bore and permanent magnet are configured such that an associated one of the force balancing coils of the proof mass assembly fits within the bore, with the permanent magnet being positioned within the cylindrical core of the coil. Current flowing through the coil therefore produces a magnetic field that interacts with the permanent magnet to produce a force on the paddle. Also provided on the inwardly facing surfaces of the stators are capacitor plates configured to form capacitors with the capacitor plates on the top and bottom surface of the paddle. Thus movement of the paddle with respect to the upper and lower stators results in a differential capacitance change.
In operation, the accelerometer is affixed to an object whose acceleration is to be measured. Acceleration of the object along the sensing axis results in pendulous, rotational displacement of the paddle with respect to the support ring and the stators. The resulting differential capacitance change caused by this displacement is sensed by a feedback circuit. In response, the feedback circuit produces a current that, when applied to the force balancing coils, tends to return the paddle to its neutral position. The magnitude of the current required to maintain the paddle in its neutral position provides a measure of the acceleration along the sensing axis.
The sensing axis of a pendulous accelerometer, such as the one described above, is perpendicular to a plane determined by the hinge axis and the center of mass of the proof mass. In nearly all practical accelerometer designs, the sensing axis is aligned with a reference axis defined with respect to the case in which the accelerometer is housed. When aligned in such a manner, the accelerometer output represents acceleration along the reference axis, and is insensitive to cross axis accelerations. High performance accelerometers must have very low cross axis sensitivities and, therefore, require close alignment between the sensing axis and the reference axis. This alignment is set by actively measuring and correcting the alignment before fixing the sensor in the case.
Force rebalance accelerometers that have only a single force balancing coil present a problem with respect to sensing axis alignment. In a dual coil design, the pendulously mounted paddle/coil unit, i.e., the proof mass, can be made symmetric, such that the center of mass of the proof mass is located halfway between the upper and lower surfaces of the paddle. In a single coil design, without a counterweight on the opposite side of the paddle from the coil, the center of mass of the proof mass is offset, such that the sensing axis is not perpendicular to the upper and lower surfaces of the paddle. The upper and lower surfaces of the paddle are typically assembled parallel to the mounting surface of the sensor case, because the paddle surfaces are relatively easy to use as references. When the sensing axis is not perpendicular to the paddle surfaces, then it becomes difficult to align the sensing axis to the accelerometer case and mounting surface.